The invention relates generally to a process for removing water-soluble impurities from the working medium of a steam turbine power plant.
In steam turbine plants using a type of steam generator in which the impurities can not be removed by washing it is necessary to prevent the impurities from reaching the steam turbine and its auxiliary components. The impurities, which enter or occur within the circulating system, such as in the cooling water or added contaminated feedwater may backflow from the unit purifying the condensation product. Instances are known where plant components were destroyed by stress-crack corrosion or other types of chemical corrosion in which impurities in the working fluid were the cause.
Published German Patent Application No. 1,178,861 discloses a known process in which the condensation product, accumulated inside the turbine between the blades and in the drains of the bleeder pipes is collected and then purified in a separate ion exchanger. Such an arrangement eliminates the need for a standard filter for cleaning the entire condensation product collected in the "hotwell".
A process in which the entire condensation product accumulating in the turbine unit upstream of the actual condenser is purified is not suitable for all types of power plants. Those plants which require the use of a unit which purifies the condensation product to supply high-quality feed water and which will recirculate up to 40% of the total condensation product accumulated within the plant cannot use the process described in the German Plant application. If all of the condensation products are centrally collected, the concentration of the impurities will not be very high. In view of the large volume of condensation products, the required purification unit must by necessity be relatively large. Costly heat-exchange surfaces are needed to cool the hot condensation product down to the purification temperature.
The main source of water-soluble impurities in the feedwater is the condensate purification unit. Depending upon the degree of exhaustion of the chemicals, a certain ionic leakage occurs through which electrolytes, particularly sodium hydroxide and sodium chloride, are introduced into the loop. Since certain steam generators (e.g., once-through steam generators in nuclear plants or once-through boilers run on fossil fuel) cannot be cleaned by a conventional blowdown process, such impurities can only be removed from the condensate by the purification unit which functions by means of an ionic exchange. Typically, the feedwater is pumped to the high-pressure loop without passing through the polishing plant so that a substantial portion of the feedwater does not pass through the condensate purification unit. Therefore, a continuous release of impurities results, with a corresponding constant increase in the concentration of impurities within the high-pressure loop.
One solution would be to eliminate the forward pumping to a condensate tank and have the condensates cascaded into the condenser and cleaned in the condensate purification unit. This process would, however, require several expensive recuperative heat exchangers in order to cool the 200.degree. C. condensate to less than 50.degree. C.
This temperature reduction is necessary because a certain decomposition of anionic exchange resin takes place at high temperatures which over a longer period of time could impair the operation of the condensate purification unit. In a cascading system, the purification unit would have to be large enough to treat the entire amount of condensate, as opposed to the recirculating system in which a portion of the condensate bypasses the purification unit. To clean about 50% more condensate an additional exchange capacity would have to be provided for, otherwise all condensates would flow through the existing purification system at a higher speed. The increased speed and greater condensate flow reduces the desired degree of cleanliness sought to be achieved and exhausts the resins faster, thereby requiring their more frequent regeneration.
Using the present invention, an already existing system can be improved as follows. The improvement is based upon the consideration that during the expansion of slightly superheated steam in the high-pressure turbine, the steam, at the point of crossing the saturation line, still behaves as if it were superheated since it is unstable. In the Wilson Line area, a steam humidity of approximately 3%, the steam stabilizes and partially condenses, resulting in the formation or water drops.
This initial condensate contains highly-concentrated water-soluable impurities (similar to the opposite phenomenon, distillation) which can be removed from the high-pressure loop before being diluted by increased condensation during further expansion of the steam. This removal takes place at the bleeding points of the turbine which are arranged downstream from the Wilson Line region. If the condensate from this bleed steam (about 13% of the entire working medium) is recirculated as before, then all impurities would remain in the loop. The bled steam contains a relatively large portion of the impurities within the circulating working medium. The bled steam is therefore passed through moisture separators which remove the condensation products from the steam. The impurities are removed from the recirculated feedwater by passing the collected condensate through the purification unit.
An object of the present invention is to provide a purification process in which already existing installations may be modified without adding costly new equipment. The addition of the condensate treatment system considerably increases the reliability of the power plant. In building new power plants, the location of the bleed steam points can be predetermined, which provides for an optimum steam generation process and enhances the separation of impurities from the condensate.